1. Field of the Invention
The invention relates to an X-ray imaging system comprising an X-ray source for irradiating an object by means of an X-ray beam, an X-ray image intensifier tube which comprises an entrance screen and an exit screen for converting an X-ray beam incident on the entrance screen into an optical image in the exit screen, a detection device which comprises a detection face which is situated opposite the exit screen in order to convert the optical image into an electric signal, a monitor which is connected to the detection device for a raster-like display of the signal as a monitor image, and an optical system which is arranged between the exit screen and the image detection face in order to image a circular optical image on the exit screen as an elliptical optical image on the image detection face.
2. Description of the Prior Art
An X-ray imaging system of this kind is known from German Offenlegungsschrift DE 36 26 532.
The cited Offenlegungsschrift describes an X-ray imaging system in which the detection device is formed by a matrix of optoelectronic transducers, for example a CCD matrix. The use of such a detection device gives rise to a problem in that the exit screen, often being circular, has a shape other than the customarily rectangular CCD matrix. In order to prevent loss of image information of the exit screen, the image of the exit screen forms the inscribed circle on the rectangular entrance screen of the detection device. As a result, the photosensitive elements at the edges of the CCD matrix are not used. When the circular optical image of the exit screen of the X-ray image intensifier tube is distorted so as to form an elliptical optical image, for example by means of an anamorphic system, a larger surface of the entrance screen of the detection device is used as the image detection face and the resolution increases in a direction of a longitudinal axis of the elliptical optical image. When an exit screen of the X-ray image intensifier tube and an entrance screen of the detection device of the same shape are used, the above problem does not occur and the optical image need not be adapted to the entrance screen of the detection device from a point of view of a resolution enhancement. Because of the shape of the entrance screen, a detection device constructed as a television pick-up tube is suitable for being coupled to the exit window of the X-ray image intensifier tube, for example by means of a fibre optical system, because the optical image of the round exit screen can be made to register with the entrance screen of the television pick-up tube. For example, for displaying the optical image on a rectangular television monitor whose sides relate as 3:4, for the image detection face use can be made of an inscribed rectangle having the same ratio of the sides on the entrance screen of the television pick-up tube. In that case information in the optical image which is situated outside the image detection face on the entrance screen will not be displayed on the television monitor. However, for medical imaging it is important that all information contained in the optical image is displayed on the television monitor; this is achieved by utilising the entire entrance screen of the television pick-up tube as the image detection face. In the television pick-up tube a positive charge image is built up on the image detection face which comprises, for example a photoconductive layer, which charge image corresponds to the optical image on the exit screen of the X-ray image intensifier tube. Using an electron beam which is deflected across the image detection face in a raster-like scanning pattern, the charge image is discharged and an electric signal is generated in the television pick-up tube. In order to obtain a high resolution, the entrance screen of the television pick-up tube is usually comparatively large in the case of medical imaging, a diameter of the entrance screen amounting to, for example 2 inches. Because of the comparatively large surface area of the image detection face, the image lines of the raster-like scanning pattern in the image detection face are spaced comparatively far apart, so that positive charge remains between the lines. When successive scanning patterns have been shifted with respect to one another, for example due to vibrations, the additional charge causes annoying flicker of the monitor image. Because the electron beam does not have the same cross-section everywhere across the image detection face, local variations occur in the monitor image due to the residual charge, which variations are not representative of the light on the exit screen of the X-ray image intensifier tube.